Interferon-gamma (IFNγ)

IFNγ, first described in 1965 as a component in supernatant derived from T-lymphocytes, was shown to augment various biological activities of macrophages 308, including intracellular killing of parasites and increased oxidative metabolism 309, enhanced expression of MHC class II antigens 310, or increased tumor cell killing 311. Since then, the 34 kDa protein IFNγ attracted much interest from clinical investigators, as it is essential for natural as well as acquired resistance to infection and cancer. Because of its immune-regulatory consequences, IFNγ was also termed “immune-interferon”

312.

1.4.2.2 The endogenous production of IFNγ

IFNγ is produced exclusively by NK cells and some sub-populations of T-lymphocytes 313,314.

Production of IFNγ by mononuclear cells was only described by the group of Fultz et al. 315. To be activated, T-lymphocytes require a combination of three signals which are (1) a specific ligand binding to the T-cell receptor, (2) a balanced assembly of cytokines, e.g. IL-1, IL-6, TNF or IL-12, and (3) contact with accessory cells through cell adhesion molecules.

1.4.2.3 IFNγ in inflammation

IFNγ release can be induced by preparations of Gram-positive bacterial compounds like Staphylo-coccus aureus 316 and Listeria 317, as well as from Gram-negative endotoxins. While the secre-tion of IFNγ in case of stimulasecre-tion by positive bacteria is restricted to NK cells, Gram-negative bacteria induce NK cells, as well as the T-lymphocytes, i.e. CD4⁺ and CD8⁺ cells to re-lease the cytokine 318. Nevertheless, in the lethal Shwartzman reaction, caused by two consecutive injections of endotoxin, the elimination of NK cells, but not of CD4⁺ or CD8⁺ T-cells, is enough to prevent the toxic manifestations of the reaction 319. While several cytokines, like TNF 320, IL-12 318, IL-2 321 and IL-1 322share synergistic effects with IFNγ, others, like IL-4 323-325, IL-10 326, TGF-β 327, 328, IFN-α and IFN-β 329, but also TNF 330were described as antagonists.

IFNγ is generally assumed to play a primordial role in the defense against intracellular bacteria and parasites 331,332. In fact, most of the pathogens are found in mononuclear cells. This IFNγ de-pendent pathway is complemented by a cytotoxic T-cell pathway, which kills phagocytes or other cells that harbour microbial pathogens 333. Furthermore, exogenously administered IFNγ has been found to act prophylactically against a variety of experimental virus infections, such as CMV infection in mice 334 or rats 335. However, in the case of HIV, the activation of monocytoid cells by IFNγ was found to stimulate rather than inhibit virus replication 336,337. Furthermore, IFNγ is well known to potentiate the respiratory burst responsiveness of macrophages to stimulants, resulting in an in-creased production of highly reactive oxidants, such as H2O2 338 and the superoxide anion 309, as well as nitric oxide (NO) 339. The production of NO again is associated with an augmented defense against bacterial infection 150,340, enhanced anti-viral effects 341 and the killing of tumor cells.

Nevertheless, macrophages activated by IFNγ have been found to have a reduced ability to ingest a variety of obligate intracellular microorganisms, e.g. Rickettsiae, Trypanosoma cruzi and Leishma-nia amastigotes 342. Furthermore, significant side effects of NO may also cause undesirable cell and tissue damage.

Another well documented action of IFNγ is the induction of MHC I and II on antigen presenting cells (APC), responsible for the recognition of viral, bacterial, tumor, transplant or auto-antigens on foreign cells, which are the preferred target for cytotoxic T-cells. Thus it was suggested that IFNγ is

crucial to allograft rejection 343. And indeed,treatment of skin allograft recipients with anti-IFNγ has been found to delay rejection of the graft 344.

1.4.2.4 Clinical significance

The application of IFNγ is controversially discussed. IFNγ was most often safe and well-tolerated, but sometimes induced severe toxicity. Furthermore, it has been reported to augment the anti-tumor effects of TNF in animal tumor models by initializing complete necrosis of tumor tissue 345. Therapy of autologous bone marrow transplantation 346, human pleural adenocarcinoma 347, ovarian cancer 348,349, colon carcinoma 350, human myelogenous leukemia 351, multiple myeloma 352, but also atopic dermatitis 353-355, furunculosis in HIV 356, visceral leishmaniosis 357,358 and Borrelia burgdorferi infection 359 with IFNγ has been shown to be safe and effective. First clinical studies were performed by Boehringer Ingelheim with rhuIFNγ (Imukin^). In case of granulomatosis, Imukin was reported to significantly reduce the risk for severe infections from 70 % (placebo group) to 23 % (Imukin group) in a dose range of 1,5 to 50 µg/kg. No toxicity, teratogenicity or side effects were found in such clinical studies 360. Furthermore, like GM-CSF, IFNγ is tested in current clinical approaches of gene therapy studies to design more selective and effective anti-cancer drugs by in-troducing cytokine genes into tumor cells 361,362. However, IFNγ was reported to play an enhanc-ing role in ischemia-reperfusion 363 and to hasten the progress of HIV infection 364.

In sum, cytokines such as GM-CSF and IFNγ are demonstrated to modulate the function of mono-cytes and have been used to experimentally probe the immunotherapeutic potential of monomono-cytes against microorganisms and malignancy. However, monocytes rarely act alone but communicate with other leukocytes involved in cell-mediated immunity. In particular, monocytes cooperate with T-helper (Th1 and Th2) sub-populations of peripheral lymphocytes. Preclinical studies in humans sug-gest that GM-CSF and IFNγ are the most promising biological response modifiers for augmenting monocyte-mediated immunity 307.

1.5 Experimental animal models of macrophage- and T-cell de-pendent inflammation

This section was initiated with the aim to describe the different animal models applied in the present work. Experimental animal models are the necessary basis for such preclinical research programs,

although extrapolation of animal studies to the clinical situation is difficult. Due to the easy handling and low costs of purchase and keeping, rodents were selected for larger study scales. In the follow-ing section, the experimental murine models, by which the effects of stimulative cytokines on different cells of the immune system were studied are described.